Solid-state imaging systems or imaging readers have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one- and two-dimensional bar code symbols, each bearing elements, e.g., bars and spaces, of different widths and reflectivities, to be decoded, as well as non-symbol targets or forms, such as documents, labels, receipts, signatures, drivers' licenses, employee badges, and payment/loyalty cards, each bearing alphanumeric characters, to be imaged. A known exemplary imaging reader includes a housing either held by a user and/or supported on a support surface, a front window supported by the housing and aimed at the target, and a scan engine or imaging module supported by the housing and having a solid-state imager (or image sensor) with a sensor array of photocells or light sensors (also known as pixels) that lie in an imaging plane and face forwardly toward the front window, and an imaging lens assembly for capturing return light scattered and/or reflected from the target being imaged through the window over a field of view, and for projecting the return light onto the image sensor along an internal optical path between the window and the image sensor, to initiate capture of an image of the target over a range of working distances in which the target can be read. The imaging lens assembly includes a plurality of lenses arranged along an imaging axis that is generally perpendicular to the imaging plane of the image sensor. Such an image sensor may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing and processing electrical signals corresponding to a one- or two-dimensional array of pixel data over the field of view. These electrical signals are decoded and/or processed by a programmed microprocessor or controller into information related to the target being read, e.g., decoded data indicative of a symbol target, or into a picture of a non-symbol target.
In order to increase the amount of the return light captured by the image sensor, especially in dimly lit environments and/or at far range imaging and reading, the known imaging reader may also have an illuminating light assembly, which also faces forwardly toward the front window, for illuminating the target with illumination light from an illuminating light source, e.g., one or more light emitting diodes (LEDs) and illuminating lenses, for reflection and scattering from the target. The known imaging reader may also have an aiming light assembly, which also faces forwardly toward the front window, for projecting an aiming light pattern or mark, such as a “crosshair” pattern, with aiming light from an aiming light source, e.g., an aiming laser or one or more LEDs, through aiming lenses on the target prior to imaging. The user aims the aiming pattern on the target to be imaged during an aiming mode prior to imaging and reading.
In the hands-free mode, the user may slide or swipe the target past the window in either horizontal and/or vertical and/or diagonal directions in a “swipe” mode. Alternatively, the user may present the target to an approximate central region of the window in a “presentation” mode. The choice depends on the type of target, operator preference, or on the layout of a workstation in which the reader is used. In the handheld mode, the user holds the reader in his or her hand at a certain distance from the target to be imaged and initially aims the reader at the target. The user may first lift the reader from a countertop or a support stand or cradle. Once reading is completed, the user may return the reader to the countertop or to the support stand to resume hands-free operation.
Thus, handheld imaging readers having a two-dimensional imager, also known as area readers, have become increasingly popular in the last several years due to their ability to scan two-dimensional symbol targets, and also to omni-directionally read one-dimensional symbol targets, and also to take pictures of non-symbol targets or forms. Yet, despite these advantages, adoption of area readers still lags behind imaging readers having a one-dimensional imager, also known as linear readers, as well as laser-based readers, because the area readers cost more and have certain performance issues, such as a smaller working distance range.
One factor that limits the working distance range of existing handheld area readers is pixel resolution of the image sensor. Pixel resolution refers to the size of the smallest detail that the image sensor can resolve (assuming focus is adequate) and is determined by the size of a detail in the image projected onto the image sensor. In the case of reading symbol targets, the image of the symbol target on the image sensor grows smaller as the distance to the symbol target is increased. When the width of the image of an element (bar or space) in the symbol target approaches (for instance) around the same size as a pixel, then the end of the working distance range has been reached, simply because if the symbol target moves any further away than that, then that element can no longer be resolved by the image sensor. The pixel resolution limits the working distance range, because the internal optical path distance between the front window of the reader and the imaging lens assembly is limited by the size of the reader housing, and causes the field of view to be unnecessarily wide.
Typically, the image sensor is positioned about 2.5 inches back from the front of the reader housing due to a practical, ergonomic requirement to build the reader housing with a size that users have become accustomed to with previous generations of reader technology. Another user expectation is that the working distance range begins very close to a front end, or nose, of the housing, because many users will naturally position the target close-up to the nose when attempting to scan the target. In order for a symbol target to be scanned close to the nose, the field of view must be able to expand to be at least about 1.75 inches in a horizontal direction across the nose. If the field of view is much smaller than that, then it may not entirely cover common symbol targets, and such symbol targets will not be read. This requirement, along with the positioning of the image sensor inside the housing, results in a need to make the field of view expand at an angle of around 40 degrees horizontally, which is typical of all general purpose handheld area readers that are available at this time.
Sometimes, it is desirable for the reader to scan a target at a distance well away from the nose, for example, when scanning a heavy or bulky item that is too difficult, or inconvenient, to lift out of a shopping cart onto a check-out counter. Thus, the handheld reader will often be called upon to read both close-in and far-out targets. With the typical field of view angle of about forty degrees, the field of view will diverge rapidly as the target distance increases, and the pixel resolution will fall off correspondingly. All area readers would benefit if the rate of divergence of the field of view could be reduced, since that would mean that the size of the image on the image sensor will not grow smaller as quickly when the target distance to the target is increased.
Aside from pixel resolution, another factor that limits the working distance range is depth of focus of the imaging lens assembly. When the imaging lens assembly is focused farther away from the image sensor, the depth of focus is increased. The working distance range may be increased simply by focusing the imaging lens assembly farther away from the nose. However, this solution has not been utilized in existing handheld readers, because it will reduce the sharpness of the image close-up to the nose, thereby preventing reading of some close-in targets.
Another performance issue with existing area readers relates to parallax involving the aforementioned aiming light assembly. Since the outgoing aiming light cannot coexist with the incoming return light projected onto the image sensor in the same place, the aiming light sources must always be positioned to one side (or above or below) the image sensor. It is desirable that the aiming light be visible close to the center of the field of view of the image sensor at a distance away from the reader. The aiming light on existing scanners is therefore directed at a slight angle with respect to an imaging axis of the imaging lens assembly, so as to converge the aiming light on the center of the field of view at a convenient distance away from the reader. Unfortunately, this results in the aiming light being off-center at all other distances, and the larger the angular difference between the aiming light assembly and the imaging lens assembly, the faster the aiming light goes off-center.
As previously mentioned, the known area readers are expensive not only in terms of component cost, but also in terms of the labor cost of assembling and aligning their various components. Unlike linear readers and laser-based readers, optical and ergonomic constraints have compelled the use of multiple printed circuit boards (PCBs) interconnected by flexible wiring, such as ribbon cables, and connectors inside the handheld area readers. The image sensor is typically mounted on one of the PCBs, while other components, e.g., the illumination LEDs or the imaging lens assembly, are mounted on another of the PCBs. In an area reader, the image sensor must be positioned in a plane perpendicular to the imaging axis of the imaging lens assembly. This imaging axis projects out of the front of the housing at an angle that is typically tilted by around fifteen to about twenty-two degrees with respect to the handle of the housing, for ergonomic reasons. In order to totally eliminate all internal flexible wiring and secondary PCBs, a single PCB must extend from a bottom of the handle (where an interface cable is connected) to a top of the housing (where success indicator LEDs for the human interface reside for visibility). The fifteen to twenty-two degree tilt of the handle forces this single PCB to be tilted by this same angle with respect to the imaging axis of the imaging lens assembly, thereby requiring that the known imaging lens system be placed on a secondary PCB tilted by that angle with respect to the main PCB. Multiple PCBs decrease reliability.
The market for handheld area readers is growing, but market growth is hampered not only by the relatively high cost of the scan engines, as driven by their complex electro-mechanical structure that employs multiple PCBs and/or ribbon cable interconnects and/or multiple connectors, alignment fixtures, etc, but also by performance issues, such as a limited working distance range due to pixel resolution constraints and depth of focus constraints, as well as by parallax issues. Accordingly, there is a need to provide a compact, low cost, high performance, and durable scan engine and an apparatus for, and a method of, electro-optically reading a target by image capture employing the compact scan engine in a handheld reader that would spur market growth.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.